This is the largest transformer I've ever personally come into contact with:

(picture is not of me, I visited this site 11 years ago while building some new studios for the station.)

That's one of two modulation transformers used by the 500,000 watt AM transmitter at WLW in Cincinnati, the highest-power broadcast radio station ever constructed in the western hemisphere. Built in the early 1930s, it's no longer in use. Each one weighed 35,000 lbs and still contains 725 gallons of PCB oil. The rest of the transmitter was massive beyond description. Rather than being constructed inside a metal box as is the custom today, it simply consumes nearly all of the two story building which houses it. The building itself is the transmitter cabinet:

(note the "Danger: High RF Radiation" sign at the front door. When you walk through that door, you are inside the transmitter.)

Back in those days, AM radio was modulated by amplifying audio signal up to about half of total transmitter power and feeding it into the primary of the modulation transformer. The secondary of the transformer was wired in series with the high-voltage power supply to the plate of the final output tube(s), the grid(s) of which were driven with the RF carrier frequency at full saturation.

They basically just smashed the two signals together and hoped for the best. Surprisingly, it usually worked. When it didn't, the transformer would sometimes explode.

Ours are dry iron core units and still weigh 20,000lbs. These are taking 13200 line voltage and converting it down into 480 for our process line. One of them powers our 3 chillers and MCC, while the other one powers our 5 rectifiers.
8000a 60v DC glycol cooled rectifier taken apart for maintenance:

This is the smaller of the 5 we have, it's only 60v instead of the bigger 80v.